Jun 26, 2013 3:39 PM

Found these on some trees in the garden in pretty large numbers. First photo is on a miniature/dwarf weeping willow and seem to be little green flies which sit all over the end of the branches. Second is an ornamental cherry tree with these things on the underside of leaves - like a brown slightly scaly looking circle sitting on white fluff. Does anyone know what these are, and more importantly what can be done to get rid of them? Reluctant to use pesticides because of birds nesting in the trees (particularly the willows).

Both are sap suckers, and can be controlled by systemic insecticides (much more restricted choice these days, since Imidacloprid (various brand names) was linked to bee collapse disorder), contact insecticides (less effective), and biological controls (difficult to be effective outdoors and on large plants).

The birds will, directly or further down the food chain, be exposed to any chemicals you use to treat these infestations. I don't know what to suggest that would be effective and bird-safe.

I agree with Mike that they're most likely aphids in photo 1. But why do anything about them? I have the same sort of willow. Every year I get aphids on it and every year the population crashes eventually due to predators, parasites and pathogens. And whilst they're there they'll be food for the birds.

From the point of view of the health of the tree, the ecology is more complicated than it might at first appear. Not only do the aphids provide food for birds and other animals (eg. lacewing and ladybird larvae), they also secrete honeydew, which may be farmed by ants, which may remove more-serious pests from the tree. Some of the honeydew that is not harvested by ants, falls to the ground - where it may help the processes of leaf decomposition (eg. providing an accelerant for fungi), thereby making nutrients available to tree more quickly/efficiently. The honeydew that falls on leaves can cause sooty mould to develop there, which is unsightly and which must reduce photosynthetic efficiency - so that appears to be a negative. But at the same time, it might be preparing the leaves for rapid decomposition once they call. Back to the birds: having been attracted by the greenfly, they'll be in prime position to pick off caterpillars and other leaf-eaters.

...Which is all by way of saying, the interrelationships are complicated (as one would expect after aeons of co-evolution), and so it might be wise to look past the obvious to see some of the deeper ecological workings.

[This statement comes with a warning: It is easy to become fascinated with nature's soap opera called ecology: by its setting, its actors and their stories - which become more entwined the longer you watch.]

Here here MIke and 'jag' - I am a great believer in just letting nature get on with it itself. I rarely have a sustained aphid problem as the varied predators find them eventually and as mike says there are other, somewhat hidden, benefits along the way. It’s a very complex web and we shouldn't tamper with it lightly, even on a small scale … you will have the ecology of your herbaceous border slewing about all over the place.